Environmental compliance in the transportation industry continues to be problematic for society. Control of emissions levels is particularly costly for the commercial ground transportation industry because Compression Ignition (CI) engines have a set of technical challenges different from Spark Ignition (SI) engines. Present and future emissions compliance demand systems advancements in diesel engine technology. Solutions increase vehicle costs and elevate maintenance costs. Another undesirable outcome which stems from compliance with NOx emissions standards relates to the further generation of greenhouse gases, as reductions in fuel efficiency have been accepted as a necessary cost of compliance with NOx emissions standards.
Ideally, optimum fuel efficiency in a diesel or gasoline powered internal combustion engine requires adjustment to a relatively high air-to-fuel ratio such that the ratio is positioned away from a relatively rich fuel content to a slightly fuel rich ratio that is relatively close to the stoichiometric ratio. FIG. 1 is exemplary. With this higher combustion efficiency there is a relatively high combustion temperature which generates a greater mechanical force than achieved at lower combustion temperatures. This results in a relatively higher power output. It is also widely acknowledged in the literature that the higher combustion temperature results in higher NOx emissions levels. See FIGS. 1 and 2. Clearly, implementing environmentally acceptable solutions for controlling NOx emissions runs counter to the air-to-fuel configurations which result in more optimal fuel efficiencies and lower CO, HC and Soot emissions.
Despite this drawback, it is widely accepted that control of NOx emissions in diesel engines must be addressed with some form of an Exhaust Gas Recirculation (EGR) system which re-uses spent combustion gases. Typically, EGR systems recirculate gases from the exhaust manifold through the intake manifold. The extent of recirculation may range from 10 percent to over 50 percent. This affects reduction in the oxygen content at the intake manifold, effectively depressing the air-to-fuel ratio. With relatively rich fuel content in the combustion chamber, the reaction is shifted further away from the stoichiometric ratio. This, in turn, reduces the combustion temperature to a level which reduces NOx generation to a more acceptable level, perhaps up to about a fifty percent reduction. However, as the level of exhaust recirculation increases, there is increased heat rejection which requires a larger cooling system. Another drawback is that with exhaust gas recirculation diluting the volume percent of oxygen entering the engine from the intake manifold, the engine power density decreases. This gives rise to a need for a larger displacement engine to achieve the same power output. Also, when the volume percent of oxygen decreases, more soot is generated and more unburned hydrocarbons are also carried out the exhaust. With regulatory limits on both particulate matter and unburned hydrocarbons it has become necessary to incorporate additional equipment in the engine exhaust system, e.g., diesel particulate filters which may remove only about eighty five percent of the particulate matter. Generally, EGR systems require additional components to overcome or offset the aforementioned drawbacks. They result in excessive engine wear and higher maintenance requirements due, for example, to entry of carbon into the motor oil.
It is also recognized that an EGR system cannot, alone, provide sufficient NOx emission reductions to comply with many current and future emissions requirements. Due to the aforementioned drawbacks of EGR systems in diesel engines, original equipment manufacturers have incorporated systems with other means to reduce NOx emissions and to even reduce the percentage of exhaust gas recirculation. Selective Catalytic Reduction (SCR) systems are exemplary. Such systems inject an aqueous solution of urea into the exhaust flow in the presence of a catalyst to convert the NOx into molecular nitrogen and water. Treatment of exhaust gases by catalytic reduction after initial NOx removal with an EGR system enables engine operations to meet current regulatory requirements; and while it is essential to incorporate exhaust gas recirculation in diesel engines to meet emission level standards, the necessary level of recirculation can be reduced with an SCR system. Ideally, alternate means for reducing the NOx emissions should completely supplant the need for EGR systems.
The simplified schematic diagram of FIG. 3 illustrates a contemporary CI engine system 1 having a diesel fueled multicylinder engine 3 having an engine control system, an EGR emissions control system and a secondary exhaust emissions control system. The emissions control systems limit exhaust levels of NOx, particulate matter and hydrocarbons. Illustrated engine components include cylinders 11 in each of which a piston 13 is positioned for movement to compress an air-fuel mixture within a combustion chamber region 15. The engine includes an air intake manifold 19 which receives pressurized air from an intake 21 via a turbocharger 23. A positive displacement pump 31 sends pressurized fuel through the fuel rail 33 to an injector 35 for each cylinder. Exhaust from the combustion chambers exits the engine through the exhaust manifold 39, the turbocharger 23 and the exhaust pipe 43. The EGR emissions control system comprises an EGR manifold 45 connected between the exhaust manifold 39 and the air intake manifold 19 to mix a percentage of the exhaust with air received into the intake 21. An EGR valve 49 positioned in-line with the EGR manifold 45 regulates the amount of exhaust being returned to the combustion chambers via the intake manifold 19.
The secondary exhaust emissions control system includes electronic controller 51, a Diesel Particulate Filter 53 and a Selective Catalytic Reducer 55, each in line with the exhaust pipe 43. Upstream of the Filter 53 there are positioned in the exhaust pipe 43 an exhaust temperature sensor 57 and a NOx sensor 59 which each provide a signal 57s or 59s only to the controller 51. An intermediate temperature sensor 61 is positioned in the exhaust pipe between the filter 53 and the Selective Catalytic Reducer 55. An output NOx sensor 63 positioned in the exhaust pipe 5 measures the NOx level in exhaust leaving the pipe 43. The intermediate temperature sensor 61 and the NOx sensor 63 each provide a signal 61s or 63s only to the controller 51.
The engine control system comprises an Electronic Control Unit (ECU) 71 which is connected to receive signals from each of an intake manifold pressure sensor 75, an exhaust pressure sensor 77, a fuel rail pressure sensor 79, a barometric pressure sensor 81 and a crank shaft position sensor 83. The ECU also sends a control signal 87 to the EGR valve 49 to regulate the amount of exhaust flow recirculated into the manifold 19 and a control signal 89 to regulate the timing and duration of the opening of the fuel injector 35.
Like reference numbers are used throughout the figures to denote like components. Numerous components are illustrated schematically, it being understood that various details, connections and components of an apparent nature are not shown in order to emphasize features of the invention. Various features shown in the figures are not shown to scale in order to emphasize features of the invention.